This is an interesting passage of scripture. Prior to the advent of Jesus of Nazareth, the Prophets alone had the SPIRIT of Christ. It was with limited illumination to say the least. When Jesus walked and talked among humanity, he alone had that Holy Spirit. However, he told his followers that it was to their advantage that he went to the Father, because when that happened, He would then send the Spirit, i.e., the Spirit of Christ to dwell in them. Let's delve into the text a little to see what was being said and to whom.
The first interesting thing that I see is this. The audience that Peter was addressing, believers in the first century, scattered out through the diaspora, were expecting a literal, physical, national salvation. However, Peter makes it clear that it is the salvation of their soul. In so doing he is referring to the breath, essence of the human person, i.e., the human's spirit. It is separate from pneuma which is the Spirit of Christ. In the latter, pheuma is used as both the Spirit of Christ and God synonymously. Somehow, the Spirit of Christ combines with the soul/human spirit to bring about salvation. It was also tied up in the relationship between the messianic sufferings and glory.
Verse 12 makes clear who the intended, addressed audience was. It was the original readers of the epistle. All of the prophets prophesied to serve them. According to verse 12, it was also announced to them by those who preached the gospel. Paul states that the gospel was a mystery in his epistles, and here, Peter tells them that the angels in heaven did not know about this salvation of the soul, and were interested in looking into it and finding out about it. The Spirit of Christ was the instrument to help bring about this salvation.
I think it is worthwhile to look into how the Spirit of Christ, the pneuma, interacts with the spirit of man the psuche. The New Testament scripture clearly teaches that the Spirit of Christ acts in conjunction with the gospel message to affect the human spirit, the soul. Here is how it works practically. When one hears the gospel of grace, i.e., that God was in Christ reconciling the world to Himself not imputing their sin to them, but rather, imputing the righteousness of Christ to them so that they would be completely righteous in His sight, the Holy Spirit convinces that the gospel message is in fact true, thereby creating peace with God the Father. In turn, peace with God the Father, creates supernatural love for the Father while one is hearing this gospel message, and believing it through the supernatural power of the Holy Spirit. This genuine love for God, based upon grace alone, affects the human soul/spirit which in turn causes the person to want to please God. So, as Paul said, it is the grace of God that leads to repentance.
You can begin to see the necessity of hearing the gospel often. The more one hears the gospel message, the more the Spirit of Christ, the pneuma, interacts with the human spirit, the psuche to act as a transforming influence. This is precisely why one is so uplifted by hearing the gospel message. It is also the reason that Paul stated that he was determined to do nothing except preach the gospel to the Corinthians. I am certain that he was determined to do nothing except preach the gospel wherever he was, but I mention the Corinthians because that was the community he revealed it too and was addressing at the time.
So, why is the gospel so important? We can look to Paul again to explain it. He explains it in Romans chapter 7. The Law of God is good and holy. It prescribes righteousness. However, it cannot produce righteousness in the psuche/soul. It is only through the pneuma/Holy Spirit combined with the hearing of the gospel that righteousness can be produced in humanity. First let me right off put to bed an error that I have heard from holiness/Pentecostal preachers. They say that Paul is describing the old man, before salvation in Romans 7. That is totally impossible. The reason is that Paul says that he delights in the Law in his inner man. Without the pneuma and the gospel, one has no inner-man with which to delight in the Law. Therefore, Paul is addressing believers in Romans chapter 7. The bottom line of that passage is that without the gospel of grace, one cannot do law righteousness. "The good I long to do, I simply cannot do, and that which I really do not want to do, I do all to readily. This is only true however when my focus is on being righteous by virtue of Law obedience.
It cannot be clearer that the Spirit of Christ, in conjunction with the gospel message is absolutely essential in our moving toward practical righteousness as individuals. So this begs the question' why is there so much insistence on Law preaching and obedience in evangelical Christianity? The only possible answer is misunderstanding and perhaps outright ignorance of the operation of the transformation process. Further to the point, a misunderstanding and perhaps outright ignorance of of the necessity of rehearsing and hearing the gospel message over, and over, and over, and over. Get this point... the seasoned saint needs the gospel as much as the unconverted sinner! Let me repeat myself... the seasoned saint needs the gospel as much as the unconverted sinner.
This post is getting lengthy so I will end here but I want to briefly add that the Spirit of Christ, after the resurrection affected the entire spiritual atmosphere, and is progressively moving even to this very day. It is bringing an age enduring, age changing dynamic today that will radically transform the future. It will catch on more and more and will manifest itself in ways not formerly known or accepted.